Advice on possible packages

Howdy,

I've been approached with an opportunity to develop a full business application for a company. It has a relatively simple product type (only a handful of items, all in the same category) and will use paypal to manage these payments. The site will also house details on staff, so it will be fairly comprehensive in this regard - the intention is for the business to be hosted on a webserver to allow for remote management since the product for sale is strictly delivery based (keeping it vague, bare with me).

My question is whether or not I should look at an existing CMS for this or build one from scratch. I frankly have had next to no experience with CMS packages as I like to program everything. The existing business site is using Joomla! but this seems like a nightmare to plug staff management functionality in to. That could simply be inexperience though.

So I ask you, oh wise and knowing community, whether or not;
- a package like Joomla! is a good idea and which ones are recommended
- they are easy to learn given a deadline of 23 days (cutting it fine, I know)
- if there are any pre-existing "business in a box" packages like this around already

Cheers